Recently, with the advance of liquid-crystal display units, color display devices using colored polarizing films to permit the arbitrary selection of a color are being developed. Among others, there is an eager demand for colored polarizing films which have excellent thermal resistance, moisture resistance, weather resistance and the like and are suitable for use under high-temperature conditions or in an outdoor environment.
Among conventional polarizing films using an organic polymer film, a polyvinyl alcohol film having iodine adsorbed therein is well known and has been used as a polarizing film usually having a neutral gray color. Polarizing films of this type can be combined, for example, with liquid-crystal display units to produce monochromatic display devices, which are being utilized in a great variety of electronic calculators, instruments and the like.
In recent years, increasing importance is attached to the fashionability and differentiability of commercial products and the diversification of displayed information, and it is correspondingly demanded to develop colored polarizing films which permit information to be displayed in any desired color. It is conventionally known that such colored polarizing films can be obtained by coloring a film of a hydrophilic polymer (such as polyvinyl alcohol or the like) with a direct dye or acid dye having a water-soluble group and then stretching it uniaxially. These colored polarizing films are used by interposing them between a pair of transparent protective coats such as triacetate resin films, acrylic resin films or glass sheets. However, as the range of use of such polarizing films is extended in recent years, it is more urgently desired to improve not only their inherent performance such as polarizing power and the like, but also their durability such as light resistance, moisture resistance, thermal resistance and the like. As a result, the above-described colored polarizing films comprising a hydrophilic polymer film colored with a direct dye or an acid dye have proved to be incapable of satisfying the requirements of the market because there is a limit to the improvement of their durability.
In addition to the colored polarizing films comprising a hydrophilic polymer film colored with a direct dye or an acid dye, colored polarizing films comprising a hydrophobic polymer film colored with a dichromatic dye having a water-soluble group or an ionic group are also known (Japanese Patent Laid-Open Nos. 106743/'78 and 51701/'81). However, these colored polarizing films have not yet been put to practical use because their polarization performance is inferior to that of polarizing films of the PVA type and/or they require a complicated manufacturing procedure.